Medium-sized perennials, hapaxanthic perennials or annual herbs, usually rosulate, often tufted, rarely caulescent, with horizontal or erect rhizomes. Indumentum of multicellular uniseriate and sometimes glandular hairs, terminal cell simple or bibranched (malpighiaceaous hairs). Leaves usually spirally arranged, rarely distichous, in some species distichous in the vegetative parts and spirally in the inflorescence or vice versa. Inflorescences forming single headlike indeterminate spikes (here called heads) on thin peduncles, or composed of many (up to 1000) such heads. Peduncles with a closed sheath at the base. Heads composed of 10 to more than 1000 unisexual flowers. Heads subtended by an involucrum (involucral bracts) different in size and shape from bracts subtending flowers; the two types connected by intermediates. Inner involucral bracts petaloid and capable of hygroscopic movements in Paepalanthus subgen. Xeractis and Syngonanthus sect. Eulepis. Flowers generally unisexual (hermaphrodite only in Rondonantus flabelliformis, Syngonanthus amazonicus and few related species), mostly monoecious, rarely and then not constantly dioecious, heterochlamydeous, actinomorphic or slightly zygomorphic, trimerous or dimerous, nearly always isomerous, rarely heteromerous (2- and 3-merous). Sepal whorl and bracts white, brownish, yellow, or dark green to black. Petal whorl generally colorless or white, often hyaline, sometimes yellow or brown when dried, rarely (Syngonanthus anthemidiflorus) petals and gynoecium yellow. Stamens usually 3 or 2, oppositipetalous, or 6 or 4 in 2 whorls (only Eriocaulon, Mesanthemum), Syngonanthus amazonicus with a single stamen. Anthers introrse, basifixed or dorsifixed, 2-thecate 4-sporangiate, in some species 2-thecate 2-sporangiate by facial reduction (Lachnocaulon, Blastocaulon, some spp. of Paepalanthus). Ovary 3- or 2-locular, each locule with a single atropous pendulous ovule. Fruit a dorsicidal loculicidal capsule. Seed coat longitudinally striated or "hairy" by residuals of anticlinal walls of the inner layer of the outer integument. Embryo small, bell-shaped, poorly differentiated, at the micropylar end laterally attached to the starchy endosperm.
Eriocaulaceae occur mainly on poor sandy soils, in marshy or boggy habitats, and in the mountains, even on rocks. Some species of Eriocaulon, Paepalanthus, and Leiothrix and Tonina grow submersed in rivers, ponds, and in temporarily flooded places. Other species, mainly of Paepalanthus, Leiothrix, and Syngonanthus, occur in the mountains on rocks or on sandy soils in habitats which appear to be rather dry; they grow together with xerophytic plants such as Trimesia and Melocactus. Several species are both annual or perennial according to climate and living conditions. The Eriocaulaceae have no spezialized hibernating organs such as bulbs or underground corms, they endure short dry periods with well-developed green leaves. Leiothrix niveus has hairy green leaves in the summer but is completely covered with white hairs during the winter, so that then only small wool cushions but no green parts of the plant can be seen. Some species seem well adapted to periodic fires (such as the Velloziaceae which co-occur with them in the same habitats) and the older parts of their elongated stem lack leaves. The water cisterns may also be important in respect to fire protection.
Eriocaulon has the widest distribution of all genera of the family and is distributed throughout the tropics and subtropics; some species enter the humid temperate regions of N America and Europe, such as E. septangulare (Ireland, Scotland, native) and E. cinereum (rice fields of N Italy, anthropogenic). In E Asia Eriocaulon occurs in the Indo-Malayan region, in China, and also in Japan, and reaches Sachalin. Syngonanthus and Paepalanthus are mainly S American and with some species are represented in Central America, Africa, and Madagascar. Syngonanthus has also a single species in the southern USA. Mesanthemum is restricted to Africa and Madagascar, Leiothrix, Blastocaulon, Philodice to Central America. Tonina occurs in northern S America, Central America, and Cuba. The species of Rondonanthus are restricted to the Guayana Shield. Lachnocaulon is restricted to the east coast and the Mississippi region of N America.
In Central Brasil (Minas Gerais and neighbouring areas) the heads of several species (mainly Syngonanthus sect. Eulepis) with their long peduncles are collected and dried for room decoration. The value of exported inflorescences, which are often coloured when marketed, has reached nearly 2 Mio.US $ (A.M. Giulietti, pers. comm.) and several species are endangered because of intensive collecting.
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